1. Field
Exemplary embodiments of the present invention relate to a Light-Emitting Diode (LED); and, particularly, to an LED having an ohmic electrode and a method of fabricating the same.
2. Discussion of the Background
In general, III-group element nitrides, such as gallium nitride (GaN) and aluminum nitride (AlN), have been considered as materials for LEDs for blue and ultraviolet light-emission areas because they may have excellent thermal stability and a direct transition-type energy band structure.
In particular, blue and green LEDs using GaN may be used in various fields, such as natural color flat display devices, traffic lights, indoor illumination, high-density light sources, high-resolution output systems, and optical communication.
The nitride semiconductor layer of such a III-group element, in particular, GaN, may be grown on a heterogeneous substrate having a similar crystal structure using a process, such as a Metal Organic Chemical Vapor Deposition (MOCVD) method or a Molecular Beam Epitaxy (MBE) method, because it may be difficult to fabricate a homogeneous substrate on which GaN can be grown.
A sapphire substrate having a hexagonal system structure is chiefly used as the heterogeneous substrate. However, an LED structure may be limited because sapphire is not electrically conductive, and processing, such as cutting and shaping, may be difficult because sapphire is very stable mechanically and chemically, and sapphire has very low thermal conductivity.
Accordingly, research has been carried out on a technique for fabricating an LED having a vertical type structure by growing nitride semiconductor layers on a heterogeneous substrate, such as a sapphire substrate, and then detaching the heterogeneous substrate.
FIG. 1 is a cross-sectional view of a conventional vertical-type LED.
Referring to FIG. 1, a conventional vertical-type LED 1000 includes a conductive substrate 1100. Compound semiconductor layers, including a P-type layer 1500, an active layer 1600, and an N-type layer 1700, are formed on the substrate 1100.
Furthermore, a P-type electrode 1400 and an adhesive layer 1200 are interposed between the compound semiconductor layers and the conductive substrate 1100.
In general, the compound semiconductor layers are grown on a sacrificial substrate (not shown), such as a sapphire substrate, using an MOCVD method.
Next, the P-type electrode 1400 and the adhesive layer 1200 are formed on the compound semiconductor layers, and a metal reflection layer 1300 may be formed between the P-type electrode 1400 and the adhesive layer 1200. Furthermore, the substrate 1100 is attached to the adhesive layer 1200.
Next, the sacrificial substrate is separated from the compound semiconductor layers using a laser lift-off technique, and thus the N-type layer 1700 may be exposed. Next, an electrode pad 1800 is formed on the exposed N-type layer 1700. Since the substrate 1100 having excellent heat dissipation performance may be adopted as described above, LED light-emitting efficiency can be improved and the vertical-type LED of FIG. 1 may be provided.
Such a vertical-type LED may adopt the P-type electrode 1400 that is subject to ohmic contact in order to reduce contact resistance between the compound semiconductor layers and the metal reflection layer 1300.
The N-type electrode 1800 is formed on the N-type layer 1700. The N-type electrode 1800 may be made of Al—Ti-based materials. When forming the N-type electrode 1800, the Al—Ti-based materials may require a high annealing temperature of 600° C. or greater. When annealing the N-type electrode 1800, thermal damage may occur in other previously formed layers. High-temperature annealing of 600° C. or greater may cause problems during the process, such as a reduction of reflectance in materials that form the P-type electrode 1400 and the deterioration of an ohmic characteristic. In particular, the Al—Ti-based materials may not form ohmic contact on the N-polar surface of a free-standing GaN substrate.
Furthermore, contact resistance may be high at a portion where the N-type layer 1700 contacts the N-type electrode 1800 because the N-type layer 1700 has a higher energy bandgap than the N-type electrode 1800, thereby raising a device operating voltage. The amount of heat dissipated may be increased owing to the high operating voltage.
Meanwhile, a process of forming an N-type electrode on a GaN substrate without using a conventional sacrificial substrate may be used. The GaN substrate may serve as a conductive substrate if Si is doped into the GaN substrate.
Particularly, an ohmic contact for the N-type electrode 1800 may be formed by depositing Ti/Al or Al/Ti on the GaN substrate and annealing the Ti/Al or Al/Ti at high temperature of 600° C. or higher. Furthermore, a semiconductor layer may be formed on a surface of the GaN substrate corresponding to a surface of the GaN substrate on which the N-type electrode has been formed. Accordingly, the process may be complicated and difficult to manufacture because a film formation process is performed after an electrode formation process.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form any part of the prior art nor what the prior art may suggest to a person of ordinary skill in the art.